? Difference between a 1971 underhood engine harness with and without tach?

[kevken1959]

Hi all. I'm building a 1971 Mach I with A/C and factory instrumentation group. I don't have the original harness. I found a good usable 1972 underdash harness from a convertible with A/C and tach and gauges. The best underhood harness I could find was a 1971 without the fog lamps and it was from an ***** light car. I know how to run the wires to pigtails for the fog lamps, so that's not a problem. I can purchase a new alternator harness for a 1971 or 1972 (not sure which to use) with factory instrumentation group and use it instead of what's in there now. My question is will the tach and gauges work inside the car without any other modifications to the underhood harness? I'm going to shoot this question over to Midlife Harness as well.

Thanks!

Kevin.

 

[midlife]

Replied via e-mail.  The 71 and 72 underdash harnesses are largely compatible with some minor variations, but most things should still work.  The 71 non-tach headlight harness will work but not contain ammeter signals.  The sportslamps need power from the dimmer switch, which means adding a line from the interior of the car through the grommet and out to the sportslamps, not an easy task but doable. 

 

[kevken1959]

Thanks Randy. If I changed the alternator feed harness to a new 1971 with tach/gauge package and added the junction block by the solenoid and used the appropriate voltage regulator would that send the signal to the ammeter? And no problems with the tach either way?

https://www.npdlink.com/product/wire-loom-alternator-feed/102024?backurl=search%2Fproducts%3Fsearch_terms%3Dwire%2Bharness%2Btape%26top_parent%3D200001%26year%3D

 

[midlife]

No.  The ammeter signals are buried in the headlight harness, and only in the tach-compatible headlight harness.  The reproduction version IS NOT tach compatible, despite what the catalog states.  Tach will work regardless of headlight harness.

 

[kevken1959]

Man. What a drag that the repro harness doesn't work for the ammeter. Can you think of a way to wire it even if it is outside the harness?

 

[Don C]

Have Rocketman convert your ammeter to a voltmeter and you won't have to try to find or try to build the shunt circuit for the ammeter.

Or ask Midlife if he has a headlight harness for the ammeter.

 

[midlife]

Don't have 1971 tach compatible headlight harnesses.  And no, I would not even attempt to wire in those wires, as they are designed such that a very small difference in resistance between two paths is what's needed to make the ammeter work.  We're talking 0.1 ohm or less difference.  I simply don't have the tools to get that fine a resistance.

 

[kevken1959]

Thanks Don C. That's a good idea. Randy (midlife), what do you think? Even if I have my ammeter converted to a voltmeter, don't I have to run 2 new separate wires to the voltmeter?

 

[midlife]

RCCI tells you how to make the changes, but it assumes one of the ammeter lines is hot; in your case it will not be.  You'll have to snag an ACC or RUN-only line to the gauge as well as a ground line (grounds already at the 3 gauge cluster). 

 

[kevken1959]

Okay. Thanks!